The Sanbagawa Metamorphic Belt is a typical high-P/low-T metamorphic belt. We studied thermal structure and microstructures within pelitic schists in the Tenryu area, Shizuoka Prefecture, Japan. Previous studies of the Tenryu area based on the Degree of Graphitization (GD) of carbonaceous materials have reported a complex thermal structure^[1] and subdivided the area into chlorite, garnet, and, in some parts, biotite zones based on mineral assemblages. However, the relationship between GD values and peak metamorphic temperature remains unclear. In this study, Raman carbonaceous material geothermometry^[2][3] and SEM-EBSD analyses were applied to investigate the thermal structure and microstructure of pelitic schists collected from the Shirakura Unit, western Tenryu area. The main mineral assemblages of the pelitic schist included quartz, albite, muscovite, chlorite, and carbonaceous material, with garnet and calcite observed in some of the samples. The temperatures estimated using Raman carbonaceous material geothermometry ranged from 329 to 458 °C, with samples from the northeastern part of the study area exhibiting higher temperatures. The mean grain size of quartz ranges from 12 to 60 µm, whereas that of albite ranges from 15 to 75 µm. A positive correlation was partly observed between the Raman temperature estimates and the GD values. The mean grain sizes of quartz did not exhibit systematic variations with temperature, whereas those of albite we correlated with temperature. These findings clarify the tectono-metamorphic characteristics of the Tenryu area in the Sanbagawa Metamorphic Belt.

References

[1] Tagiri et al. (2000) Island Arc, 9, 188–203. [2] Aoya et al. (2010) Journal of Metamorphic Geology, 28, 895–914. [3] Kouketsu et al. (2014) Island Arc, 23, 33–50.

How to cite: Harada, A., Kouketsu, Y., and Michibayashi, K.: Thermal Structure and EBSD Microstructural in Pelitic Schists of the Sanbagawa High-P/T Metamorphic Belt, Japan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1000, https://doi.org/10.5194/egusphere-egu26-1000, 2026.

What resemble epidote-mineralized joints in the mid-crustal Late Cretaceous Mount Stuart Batholith (Washington, USA) may instead be evidence for mode I/joint-like fractures in the magma during crystallization. Zones ranging in thickness from a few mm to several cm are occupied by epidote, chlorite, and other minerals. Adjacent to these zones in the hornblende quartz diorite, textures suggest epidote grew into the crystal mush, and was partly replaced and overgrown by magmatic plagioclase. The plagioclase appears to have nucleated on the epidotes, and commonly contain fragments in optical continuity. The plagioclase is nearly pure albite (~An99), and Sr and Y commonly exceed Ca. These relationships do not resemble low T post-magmatic alteration of the plagioclase.

In the same areas, rounded to amoeboid patches, up to several mm across, are composed of small (25-100 micron) and uniform ‘pills’ of hemispherical or radiating chlorite aggregates (xMg~0.7) occur, in sharp contact with adjacent minerals. Some are completely enclosed inside other minerals including quartz and plagioclase, whereas others are interstitial to large igneous minerals. Some contain grains of apatite, titanite, or fragments of epidote. These are tentatively interpreted to have originated as melt patches, from a melt greatly depleted in Ca, Si, Na, and K from the crystallization of plagioclase, quartz, and other minerals, and likely in the process of crystallizing apatite and titanite. A few ultra-fine-grained patches suggest a possible glass precursor. Assuming simple hydration, it would have been a very mafic residual melt.

Complex intergrowths of minerals are common, including a partial replacement of epidote by hornblende. Fluid inclusions are large and abundant in multiple minerals.

There is abundant evidence of halogens, including fluorapatite, and Cl, F, Br, and even I detectable in several minerals. REEs are detectable even by EDS in several minerals. K-feldspar contains up to 1.5 wt% Ba.

Large, healthy chlorite crystals comparable in size to the magmatic minerals occur in sharp contact with other minerals, and these do not appear to be replacing anything, nor does chlorite like this occur elsewhere in the batholith.

A 0.3 mm zircon contains a small ovoid patch containing quartz, K feldspar, and a more calcic plagioclase. This is interpreted as a melt patch of an earlier composition, crystallized into a nanogranite.

Tentatively, these relationships suggest fracturing of the crystal mush during crystallization led to a water- and halogen-rich ‘dike’ that interacted with the adjacent melt, dropping the solidus. Epidote crystals, some multi-cm in length, penetrated the melt, but was subsequently partly resorbed during crystallization of hornblende, plagioclase, and quartz, and even possible magmatic chlorite, at a depressed solidus temperature, and remaining melt quenched into the pockets. Fluid inclusion work and geothermometry is in progress.

How to cite: Magloughlin, J.: Evidence for magma fracturing, solidus depression, coarse magmatic epidote, devitrified and nanogranite melt pockets, and possible magmatic chlorite in the mid-crustal Mount Stuart Batholith, USA, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2259, https://doi.org/10.5194/egusphere-egu26-2259, 2026.

The South Atlantic Transect (SAT) is a multidisciplinary scientific ocean drilling experiment designed to investigate the evolution of the ocean crust and overlying sediments across the western flank of the Mid-Atlantic Ridge (Coggon et al., 2024). The SAT comprises International Ocean Discovery Program (IODP) Expeditions 390 and 393, built on engineering preparations during Expeditions 390C and 395E. It targeted six sites on 7, 15, 31, 49, and 61 Ma ocean crust to sample intact in situ crust regarding crustal age, spreading rate, and sediment thickness and to investigate the hydrothermal interactions within the aging ocean crust.

An integrated petrological, geochemical and microstructural study unravels the conditions of host rock alteration and the formation conditions of mineralization within hydrothermally formed veins and voids.

This contribution focuses on the internal microstructure of hydrothermal veins in drill cores sampled during IODP Expeditions 390 and 393. Microstructures, preferably in calcite, were analyzed using Electron Backscatter Diffraction (EBSD). Here, data on the density and misorientation of calcite sub-grains potentially allow the assessment of intraplate stresses and stress variations with depth and distance from the Mid-Atlantic Ridge, related to the increasing density of crust with cooling and age.

Microstructures in vein calcite are characterized by the formation of sub-grains, indicating that calcite deformation is mainly characterized by dislocation glide. Mechanical twinning is very subordinate and does not substantially contribute to internal deformation. The evaluated misorientation axes between the calcite sub-grains indicate that basal and prism planes are the main intracrystalline gliding planes. The activation of these slip planes requires relatively high differential stresses, which are far above the critical stresses for twinning. Analysis of average calcite sub-grain sizes shows a general trend characterized by a continuous decrease in sub-grain size with decreasing distance from the mid-ocean ridge.

Oxygen stable isotope data from vein calcite indicate low precipitation temperatures in the range of 2° to 10 °C, without a correlation between precipitation temperature and the age of the oceanic host rock, and with a very minor influence of magmatic fluids. Therefore, we assume that vein calcite precipitated from seawater.

The microstructural and stable isotope data imply that several calcite veins formed in situ at the drilled sites. The microstructures, particularly the calcite sub-grain sizes, seem to indicate that the related differential stresses decrease with increasing distance from the Mid-Atlantic Ridge. This can be related to the higher cooling rates of the oceanic host rocks situated closer to the Mid-Atlantic Ridge; higher cooling rates presumably generate higher internal stresses due to higher rates of density increase and volume loss with cooling. Alternatively, it may also be related to the fact that the ridge-push forces, and therefore the related intraplate stresses, decrease with increasing distance to the Mid-Atlantic Ridge.

References:

Coggon, R.M. et al., 2024. South Atlantic Transect. Proceedings of the International Ocean Discovery Program, 390/393: College Station, TX (International Ocean Discovery Program). https://doi.org/10.14379/iodp.proc.390393.101.2024

How to cite: Kurz, W., Eckstein, L., Auer, G., Kunkelova, T., Müller, T., and Gätjen, J.: Calcite vein microstructures along the South Atlantic Transect: Implications for intraplate stress variations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5180, https://doi.org/10.5194/egusphere-egu26-5180, 2026.

The best example of optically anisotropic, sector-zoned garnet has just been discovered in stilpnomelane-garnet ironstones from Laytonville Quarry. The analyzed sample was metamorphosed under low-T (<400 °C) blueschist facies conditions, and consists of garnet and stilpnomelane in similar amounts, with minor quartz and accessory titanite, apatite, sulfides and ilmenite.

Garnet is subhedral to euhedral and < 300 µm in diameter. It locally shows rim dissolution and replacement by stilpnomelane. Optically, all garnet crystals show a weak birefringence even under conventional crossed polars. Analysis by polychromatic polarization confirms the already known sector-zoned pattern of birefringence, with six pairs of opposed pyramidal sectors displaying equal optical orientation. The twelve pyramids define the overall rhombic dodecahedral shape of the crystals. The optical sector zoning is accompanied by a subtle oscillatory concentric zoning, more developed at crystal rims where chemical zoning in Fe and Mn is strongest.

Optical measurements reveal that the birefringence in this garnet is 0.00053, and indicate that the optic axes in each sectors are oriented tangentially and form angles of 90° and 60° to each other.

Chemically, garnet displays regular concentric growth zoning with a well-developed bell-shaped Mn profile, but with a reversal at the rim. Considering all iron as FeO, typical compositions are: core = Sps₄₃Alm₄₁Grs₁₅Pyr₀₁ and rim = Alm₆₁Sps₂₀Grs₁₇Pyr₀₂. Notably, the pyrope content is extremely low, and X_Mg is < 0.03. The chemical zoning has no relationship with the optical sector zoning. Rather, some steps in the chemical zoning profile overlap with the optical concentric oscillations.

Ferric iron in the garnet was measured by electron microprobe using the flank method: the Fe³⁺/Fe_tot is in the range 5-8 %. It follows that the andradite component is not negligible, and decreases the grossular content, so that the compositional zoning becomes: core = Alm₃₉Sps₄₃Grs₁₃Pyr₀₁Adr₀₃ and rim = Alm₆₀Sps₂₁Grs₁₁Pyr₀₂Adr₀₇. Therefore, the garnet can be classified as spessartine in the inner core, and almandine in the rest of each crystal. The measured Fe³⁺ content of garnet has been used in the subsequent refinement of the crystal structure.

Transmission FTIR spectra recorded from garnets show weak absorption in the OH-stretching region, suggesting garnets contain trace amounts of OH and no molecular H₂O. However, as garnets contain numerous inclusions, it is not clear whether the observed OH-signal is due to garnet or OH-bearing mineral inclusions.

Analysis of the blueschist-facies rocks from the Laytonville Quarry deepens our knowledge of non-cubic Fe-Mg-Mn-Ca garnets increasingly observed in low-T metamorphic rocks, and allows discussion of the relationships of (non) parallelism among their optic and crystallographic axes.

How to cite: Cesare, B., Lorenzon, S., Biagioni, C., Nestola, F., Hezel, D. C., Kohn, M. J., Shopa, M., Day, M., Pamato, M., and Mugnaioli, E.: Almandine-1O and spessartine-1O in the Franciscan blueschists from Laytonville Quarry, northern California: petrographic, optical and compositional features, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5807, https://doi.org/10.5194/egusphere-egu26-5807, 2026.

At plate boundaries where continents collide, felsic continental crust can be buried to depths of  > 100 km resulting in the formation of ultra-high pressure (UHP) minerals such as coesite, a high-pressure polymorph of SiO₂. While the burial and subsequent exhumation of buoyant continental crust poses interesting questions for large-scale tectonics, the identification of such UHP terranes is difficult as few petrological barometers are suitable for dominantly felsic lithologies. In such cases, burial to extreme depths is commonly identified through the preservation of coesite or from parallel or radiating columnar grains of quartz assumed to have formed as quartz transforms from coesite—a microstructure termed 'palisades'. However, coesite readily transforms to quartz upon exhumation, while palisade microstructures can easily be modified by annealing during exhumation, meaning that UHP metamorphism of felsic lithologies may often be overlooked. Recent studies proposed that the former presence of coesite could be identified through an orientation signature inherited by quartz, providing a crucial and relatively simple test of deep subduction. However, debate exists within the literature as to whether the quartz↔coesite transformations involve specific crystallographic relationships. Before using crystallography to identify UHP terranes in nature, a better understanding of the coesite-to-quartz crystallographic signature and the conditions under which it forms is required. 

We collected crystallographic data using electron backscatter diffraction (EBSD) on quartz in rocks from the Tso Morari Complex (NW Himalaya) and the Dora Maira Massif (Western Alps), two areas known to reach UHP conditions. We demonstrate that neighbouring domains of quartz commonly feature an 84 ± 4° rotation of [c] axes around the pole of a common {m} plane, matching the rotation axis and angle of a Japan Twin. This orientation relationship is a product of epitaxy, whereby the Japan twin plane in quartz nucleates on the (b) plane in coesite. In supercell simulations, the nucleation of Japan twins can be explained by the energetically favourable alignment of quartz tetrahedra on parental coesite tetrahedra. Through subsequent high-pressure, high-temperature experiments, we demonstrate that this microstructural signature emerges over a broad range of conditions, regardless of the availability of nucleation sites (e.g., grain boundaries) or the density of crystal lattice defects (e.g., dislocations). In addition, Japan twins are present in all experimental specimens that traversed the quartz↔coesite phase boundary, whereas palisade microstructures are largely absent. Our crystallographic method of identifying UHP terranes is therefore more robust, and remains applicable even in the absence of palisade quartz. Overall, this work provides a new tool to quantitatively and unambiguously identify UHP terranes, even when all coesite has transformed to quartz. 

How to cite: Goddard, R., Cross, A., Lloyd, G., Breithaupt, T., V.Dyck, B., Chen, H., Parsons, A., and Bidgood, A.: Decoding crystallographic orientations: how grain-scale textures can be used to infer UHP conditions in felsic rocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8142, https://doi.org/10.5194/egusphere-egu26-8142, 2026.

Occurrences of anhydrous garnets enriched in almandine-spessartine-grossular component showing sector-zoned birefringence are increasingly reported in low-grade metamorphic rocks in localities around the world [1, 2]. Despite several proposed hypotheses [1, 3, 4], the origin of optical anisotropy in garnet have remained unclear for a long time. Recently, reduction from cubic to orthorhombic symmetry due to Al-Fe³⁺ ordering in octahedral sites has been demonstrated to cause the birefringence of garnets from the Cazadero blueschists (Franciscan Complex, USA), strengthening the idea that garnets directly grew non-cubic in low-T metamorphic environments [1, 2].

Stilpnomelane-garnet metasediments from Laytonville Quarry (Franciscan Complex, USA), equilibrated at T < 400°C, contain a new, especially illustrative example of direct growth of non-cubic, low-T, almandine-spessartine-grossular solid solution garnets. Garnets in these rocks show optically well-defined sector zoning under polychromatic polarizing light, not corresponding to any chemical zonation. Chemically, these crystals have a spessartine-rich core, typical Mn bell-shaped distributions, and almandine-rich rims.

Crystal structure refinements, performed on grains separated from single birefringent sectors (n = 2) analyzed by single-crystal X-ray diffraction (SCXRD), determined these garnets as I2/a12/d orthorhombic (Fddd unconventional setting) with pseudo-tetragonal (which is in turn pseudo-cubic) unit-cell parameters (c > a, b; c -a = 0.002 to 0.005 Å). Slight cation ordering between Al and Fe³⁺ within octahedral sites leads to reduction from cubic to orthorhombic symmetry, where Y1 and Y2 sites are occupied by 5% and 1% of Fe³⁺, respectively. This result is supported by cation-anion bond distances, which are longer in cation sites with greater Fe³⁺, and by EPMA data processed by flank method, which indicates Fe³⁺/ΣFe of ~5 to 8% in these almandine-rich garnets. These observations further support symmetry lowering as the cause of the optical anisotropy.

Overall, the present crystallographic investigation on Laytonville Quarry samples confirms the results obtained in garnets from Cazadero blueschists [2], reinforcing the idea that common almandine-spessartine garnets grow non-cubic at low-T conditions due to a small but non-negligible andradite component coupled with Al-Fe³⁺ octahedral site ordering. Our results recommend reassessment of garnet thermodynamics properties and urge a revision to the nomenclature and classification of this key mineral in the lithosphere, in agreement with current IMA – CNMNC rules [5]. Therefore, we propose to name the orthorhombic Fe- and Mn-rich garnet end-members as almandine-1O and spessartine-1O, and to distinguish them from their cubic analogues (almandine-1C and spessartine-1C).

[1] Cesare B et al. (2019) Sci Rep 9: 14672

[2] Lorenzon S et al. (2025) EGU2025

[3] Griffen DT et al. (1992) Am Min 77: 399-406.

[4] Hofmeister AM et al. (1998) Am Min 83: 1293-1301

[5] Nickel EH and Grice JD (1998) Miner Petrol 64(1): 237-263

How to cite: Lorenzon, S., Mugnaioli, E., Biagioni, C., Hezel, D. C., Nestola, F., Kohn, M., and Cesare, B.: Almandine-1O and Spessartine-1O in the Franciscan blueschists from Laytonville Quarry, northern California: crystallographic features, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17406, https://doi.org/10.5194/egusphere-egu26-17406, 2026.

Quartz pebbles in the conglomerates of the Devonian Furnas Formation in the central uplift of the 40 km Araguainha impact structure, Brazil, are pervasively crosscut by shear fractures and shocked, showing sets of planar fractures (PFs), planar deformation features (PDFs) and feather features (e.g., von Engehardt et al., 1992). A recent reinvestigation of such pebbles interpreted the shear fractures as resulting from post-shock brittle deformation, but still impact-related (King et al., 2025).

A petrographic thin section and a polished mount from one of such pebbles were here investigated by Universal-stage (U-stage), cathodoluminescence (CL) and electron backscatter diffraction (EBSD) to characterize the shear fractures. The shear fractures are filled by angular clasts of quartz embedded in a fine-grained quartz matrix, showing neither crystallographic preferred orientation nor recrystallization. The host quartz is twinned and clearly dislocated along the shear zones. A network of thin pressure-solution lines occurs at ca. 60° from the main set of shear fractures. The distribution and orientation of (shock) planar fractures was compared with the orientation of the shear fractures, following the suggestion that they should show a preferred orientation, and this might be related with the shock wave propagation direction (e.g., Pittarello et al., 2020). Further investigations on oriented pebbles are planned to better constrain their deformation history in relation with the impact event.

Engelhardt v. et al. (1992) Meteoritics 27:442-457.

King et al. (2025) Meteoritics & Planetary Science 60:124-132.

Pittarello et al. (2020) Meteoritics & Planetary Science 55:1082-1092.

How to cite: Pittarello, L., Hauser, N., Bonato, E., Pivato, R., Savastano, L., and Hana, T.: A shocked quartz pebble from the Araguainha impact structure, Brazil, investigated by U-stage, CL, and EBSD , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12222, https://doi.org/10.5194/egusphere-egu26-12222, 2026.

Major geodynamic processes driving Archean cratonic growth remain enigmatic, but rocks formed during Earth’s early evolution provide critical constraints on Archean geodynamics. The Rengali Province (RP) of eastern India is an Archean terrane situated between low-grade Archean rocks of the Singhbhum Craton (SC) to the north, the Neoproterozoic granulites of the Eastern Ghats Mobile Belt (EGMB) to the south, and the Archean high-grade rocks of the Bastar Craton (BC) to the west. The RP is bounded by the Barakot–Akul Shear Zone (BSZ) along its northern margin and the Kerajang Shear Zone (KSZ) to the south. As the RP provides a window into the cratonic growth processes, tectonic characterization of this province is essential. Previous studies have variably interpreted the RP as either an exhumed lower-crustal root of the SC, with the BSZ representing a thrust boundary or as a rotated fragment of the BC juxtaposed against the SC along a strike-slip contact. To resolve these contrasting models, we integrate field observations with petrology, microstructural analyses by electron backscatter diffraction, thermobarometry and geochronological investigations. The province comprises two contrasting metamorphic lithounits: (1) a southern high-grade unit composed of migmatites, augen-gneiss, charnockite, and metabasics; and (2) a northern low-grade unit consisting of actinolite schists and quartzites. Integrated field observations, petrography, and microstructural analyses indicate that high-grade and low-grade units followed distinct tectonometamorphic histories prior to their juxtaposition. Following gneissosity development, the high-grade unit records two generations of folding, followed by N–S-trending sinistral shearing event that overprinted the earlier foliation forming dome-and-basin structure. However, the low-grade unit preserves a single folding episode. Both units subsequently experienced an ESE–WNW-trending dextral shearing event at ~490–470 Ma under greenschist-facies conditions activating prism <a> slip-system of quartz, suggesting their juxtaposition during Gondwana assembly. Field-based kinematic indicators and vorticity analyses further demonstrate that the BSZ and KSZ represent dextral strike-slip shear zones. Thermobarometric calculation on the charnokcite suggests an average P-T conditions of ~750°C and ~5 kbar, while the low-grade records greenschist-facies condition evident from stabilization of actinolite+chlorite+albite+epitode+quartz±sphene in actinolite schist. Metabasics from the high-grade unit records ferropargasite+anorthite+quartz composition, indicative of upper amphibolite facies conditions. Published P–T estimates from the BC closely resemble those of charnockitic units of the RP and the metabasic rocks of high-grade unit exhibit geochemical affinities with BC metabasics. These correlations suggest that the high-grade unit of the RP is Bastar-affiliated, while the low-grade unit represents a fragment of the SC. The kinematics and P–T conditions of shear zones within and surrounding the RP closely resemble the Mahanadi and Cauvery Shear Zone. These similarities imply that, prior to Gondwana assembly, high-grade and low-grade lithounits of the RP evolved independently and were subsequently translated by ~1000 km into their present juxtaposition. This large-scale displacement was partitioned across multiple dextral shear zones farther south, accommodating small-circle motions. These shear zones can be correlated with the dextral strike-slip shear zones in East Antarctica, suggesting that this crustal-scale shear system extends beyond the Indian Shield and likely continues into the Antarctic interior.

How to cite: Debnath, A., Chatterjee, S., Paul, I., Karjini, S. S., Panigrahi, A., Gupta, S., and Buisman, I.: The Rengali Province, east India: a result of the Singhbhum Craton’s southward accretion or a ~1000-km dragging of the East Indian Shield? , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17026, https://doi.org/10.5194/egusphere-egu26-17026, 2026.

Melt inclusions in minerals provide key insight into mineralisation and petrogenesis in the lithosphere. For decades research commonly focused on glassy inclusions hosted in igneous minerals, helping to characterise a variety of magmatic processes. More recently, inclusions of anatectic melt in high grade metamorphic rocks increasingly gained more interest, providing a direct snapshot of crustal melting processes. In most cases they occur as polyphase crystalline inclusions in metamorphic minerals, consisting of quartz (or quartz polymorphs), mica and feldspar polymorphs, referred to as nanogranitoid. Nanogranitoids must be re-melted to a homogenous glass by recreating their original confinement conditions with subsequent quenching for a complete geochemical analysis.

In this project we analyse nanogranitoid-inclusion (NI) hosted in monazite and garnet from granulite facies gneisses with differing P-T-t paths of the southern Moldanubian Bohemian Massif. Distinct zonation in monazite and garnet were used to reveal this multiple complex metamorphic evolution from ca. 370 Ma to 312 Ma (Sorger et al. 2020). NI within these distinct mineral generations enable us to correlate metamorphic conditions during anatexis. For successful re-homogenisation of NI while ensuring individual grain recovery, we apply a modified established experimental routine with a piston cylinder apparatus (Bartoli et al. 2013). NI identification, textural and geochemical analysis are carried out by Raman and infrared spectroscopy, electron microscopy and laser ablation inductively coupled plasma mass spectrometry. Monazite as an important reservoir for LREE, Th, U and Y, paired with nanogranitoids provides an ideal toolset for studying partitioning mechanisms under variable conditions in natural systems. Geochemical analysis of the hosted melt inclusions could help us to further enhance our understanding of the dynamics and timing of crustal melting during orogeny, the fluids involved and the conditions of melt-host stability.

References
Bartoli et al. (2013). Geofluids 13 (4), 405-420
Sorger et al. (2020). Gondwana Research 85, S. 124–148

How to cite: Günzler, D., Sorger, D., Müller, T., Willbold, M., and Ferrero, S.: In-situ analysis of nanogranitoids in monazite opens up new paths for understanding crustal melting, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9568, https://doi.org/10.5194/egusphere-egu26-9568, 2026.

During their continuous cooling and differentiation, granitic magmas progressively shift from a medium dominated by crystal-melt interactions to a medium that is dominated by crystal-fluid interactions. We refer to this transition between a purely magmatic to a hydrothermal system as magmatic-hydrothermal transition (MHT), which is often associated with the formation of ore mineralisation. In highly differentiated and volatile-rich magmas (e.g., rare-metal granites and pegmatites), the circulation of hydrothermal fluids often modifies the original rock texture, by dissolving and/or replacing the primary minerals phases by secondary ones. Since most of our petrological interpretations are based on mineral composition (e.g., chemical zoning, geochronology), it is crucial to evaluate and quantify the chemical and mineralogical changes such rocks have undergone during the MHT.

To better understand the MHT in highly evolved magmas, especially how these episodes of fluid circulation impacted the texture and composition of the primary mineral phases, we have investigated the internal texture and chemical composition of heterogeneous zircons from the Beauvoir rare-metal granite (Massif Central, France). By combining µ-Raman spectroscopic, mineralogical and geochronological analyses on these grains, we show that primary (magmatic) zircon was partially replaced by secondary (hydrothermal) porous “zircon” through dissolution-reprecipitation mechanisms. The zircon-fluid interactions were notably facilitated by the primary, high trace element content in zircon (especially for U). This newly formed mineral grains (secondary “zircon”) are extremely enriched in non-stoichiometric elements up to few weight percent of P, U, F, Ca, Fe and Mn while they are depleted in Si and Zr compared to pristine zircon. These drastic compositional changes during the MHT of the Beauvoir granite clearly indicate that altered, secondary pseudomorphs after magmatic zircon can be a good tracer for the MHT in evolved silicic systems.

As a result of these dissolution-reprecipitation processes, these zircon grains are porous and highly metamict from the high degree of decay damage related to percent levels of Uranium, which considerably limits their use for zircon petrochronology. By comparing the ID-TIMS geochronological analyses performed on these zircon (312 ± 7.2 Ma – discordia upper intercept) with those performed on apatite (313.4 ± 1.3 Ma – ²⁰⁶Pb/²³⁸U weighted mean age, 9 analyses), we thus envision that the use of zircon to precisely date the emplacement of highly differentiated magmas is limited, while that of other minerals such as apatite (and potentially columbo-tantalite, cassiterite) may be more appropriate in such systems.

How to cite: Bouilhol, P., Esteves, N., Schaltegger, U., Ovtcharova, M., Navin-Paul, A., and France, L.: The magmatic-hydrothermal transition record in zircon: implications for zircon texture, composition and rare-metal granite dating (Beauvoir granite, French Massif Central), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17749, https://doi.org/10.5194/egusphere-egu26-17749, 2026.

The Lavrion metallogenetic district, located in the Attico-Cycladic Massif, Greece, hosts an evolving proximal calcic Fe-skarn followed by distal sulfide-rich skarn, transitioning into the world-class Pb-Zn-Ag carbonate-replacement deposit. This study investigates the structural and microstructural controls on skarn metasomatism and its transition to carbonate-replacement mineralization, with emphasis on the role of extensional tectonics and detachment faulting.

The study integrates detailed structural mapping with microstructural and mineralogical analyses of thin sections. In detail, the study included microtectonic analysis, fluid inclusion microthermometry, EBSD, SEM-EDS, stable and radiogenic isotopes, and geochronological constraints (U-Pb, Re-Os, Pb-Pb). These datasets allow us to reconstruct deformation conditions, fluid evolution, and the timing of fault-related mineralization.

The Lavrion skarn system occurred within the footwall of the Lower Tectonic Unit, where NW–SE-trending brittle to ductile-brittle faults and the West Cycladic Detachment System (WCDS) generated an extensive damage zone characterized by intense fracturing, brecciation, and enhanced permeability. Skarns, skarnoids, and associated oxide and sulfide ores are spatially localized along these fault-related structures, which acted as infiltration paths for magmatic-hydrothermal fluids.

Microstructures in the different skarn zones, i.e., garnet-clinopyroxene and garnet-epidote, and associated ores, i.e., magnetite, pyrrhotite, and chalcopyrite, including oscillatory and sector zoning, sigma-type tails, replacement fronts, crack-seal textures, and mineralized breccias record episodic fluid flow, fluctuating redox conditions, and syn-tectonic mineral growth. Prograde Fe-skarn assemblages formed at ~560–530 °C and ~0.2 GPa under relatively oxidizing conditions, leading to widespread magnetite ores. Subsequent cooling to ~460–380 °C, combined with variations in fO₂ and fS₂, acid and saline fluids, promoted extensive retrograde replacement of magnetite by sulfide ores, i.e., pyrrhotite, galena, sphalerite and chalcopyrite.

Fluid inclusion microthermometry and stable and radiogenic isotopes indicate that the skarn-forming fluids were primarily magmatic in origin and were sourced from the Miocene Plaka and Villia granitoids. The ore fluids were significantly modified through wallrock-fluid interaction with the metasedimentary host rocks within the detachment damage zone.

The WCDS, not only controlled skarn formation but also exerted a first-order influence on the temporal and spatial development of the overlying Pb-Zn-Ag carbonate-replacement deposits. The Lavrion district represents a structurally controlled calcic Fe-skarn transitioning toward the carbonate-replacement deposit, where extensional faulting, folding, detachment-related damage zones, and microstructural evolution governed fluid pathways and ore deposition. Our results highlight the importance of integrating microstructural analysis with tectonic architecture in exploration modeling for skarn-oxide-related and carbonate-replacement-sulfide related deposits in extensional or post-collisional settings.

How to cite: Fitros, M., Tombros, S., Simos, X., and Kokkalas, S.: Transition from Fe-skarn to carbonate-replacement deposits: Evidence for structural control in the world-class Lavrion Mining District, Greece., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19616, https://doi.org/10.5194/egusphere-egu26-19616, 2026.

The nature of the relationship between volcanic and plutonic rocks is a topic of ongoing debate despite decades of research. Plutons have been interpreted either as “failed eruptions” equivalent to erupted material; as “crystal graveyards” left behind after melt extraction; or as genetically distinct from volcanic rocks. Extensive geochemical, geochronological, and modelling work has not led to a conclusive resolution; at the same time, comparative textural studies of plutonic and volcanic crystal cargoes are rare despite their potential to reveal petrogenetic differences.

Here, we examine differences in plagioclase textures in volcanic versus purely plutonic rocks across a range of magma compositions and tectonic settings. We target plagioclase, an abundant igneous mineral phase which records significant disequilibrium – caused by, e.g., magma recharge and remobilisation – as prominent resorption horizons. The number of major resorption horizons was counted for ≥100 plagioclase crystals (>100 µm) per sample using BSE images, excluding oscillatory zoning and outermost rims in volcanic crystals. We observe that plagioclase cargoes in volcanic rocks consistently show more major resorption horizons per crystal (mean ≈ 4) than those in plutonic rocks (mean ≈ 1–2). This pattern is reproduced across magma compositions, except basalts, in which plagioclase crystals have a similar number of resorption boundaries (mean ≈ 1) to those in plutonic rocks.

Our results demonstrate that intermediate and silicic volcanic rocks record pronounced disequilibria more often than plutonic rocks of comparable composition, implying fundamental differences in magma storage. Assuming that major resorption horizons record recharge/remobilisation events, the observed textural contrast suggests that plutonic systems experience lower magma recharge rates and limited interaction between distinct batches, whereas higher recharge rates in volcanic systems repeatedly remobilise stored magma and promote the formation of hybridised, eruptible reservoirs. Our results highlight the potential of comparative crystal textural analysis to reassess the plutonic–volcanic connection across tectonic settings.

How to cite: Mangler, M. and Gordon, C.: Plagioclase textures reveal contrasting magma storage conditions for plutonic versus volcanic rocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20822, https://doi.org/10.5194/egusphere-egu26-20822, 2026.